In a conventional current limiting fuse, a fuse element is surrounded by a granular inert material of high dielectric strength, such as sand or finely divided quartz. The fuse element is typically wound onto a support core, or spider. When the element is subjected to a current of fault magnitude and vaporizes, arcing of the current results. The metal and the metal vapors expand into the spaces between the inert material. Contact between the hot arc and the relatively cool granules causes a heat transfer from arc to granules. This dissipates the arc energy but causes very little pressure buildup within the fuse structure. A high resistance results from the vaporization of the fuse element, the removal of metal vapors from the arc path, and the rapid cooling of the arc. The current is limited accordingly.
The resistance increases rapidly. The sand particles in close proximity to the arc become partial conductors at the high temperature of the arc. However, fused particles cool on extinction of the arc and solidify into a fulgurate, which is similar to a glass body and is an insulator.
In order to insure against surge voltage peaks in excess of predetermined limits, the fuse element may have a variable cross sectional area. It has areas of reduced cross sectional area between areas of large cross section. This arrangement can be accomplished by, for example, punching a series of circular perforations in a ribbon fuse. The edges of the perforations form the areas of reduced cross section. These areas of reduced cross section cause progressive insertion of arc resistance into the circuit. In turn, this keeps the rate of current change relatively low. As a result, this tends to limit the inductive surge voltage which may occur when a current limiting fuse operates, which otherwise might damage circuit insulation.
A gassing current limiting fuse is an improvement over that described above. In such a gassing fuse, the spider which supports the main fuse element, or at least the portions of the spider which contact the main fuse element, are made of a material which generates an arc quenching gas when contacted and eroded by the heat from an arc. Such materials include, for example, molded thermosetting compositions which include a water insoluble binder and an anti-tracking substance such as hydrates or oxides of aluminum and magnesium. The material can also include various fillers, for example, mica, glass, fiber, asbestos or silicon. One suitable material comprises about 75 percent aluminum hydrate filler, 20 percent polyester resin, and 5 percent glass fiber. See U.S. Pat. No. 3,294,936 at col. 3, lines 43 et seq.
The gas which is generated upon erosion by the arc cools the decaying fuse element and fulgurate, and also provides a high resistance to further current flow. In this manner high current flow which could damage the circuit is impeded, and heat is dissipated. As in a conventional fuse, the fuse element is surrounded by sand or finely divided quartz which forms a fulgurate when exposed to an arc and further aids in limiting the current.
The spider in gassing fuses is conventionally formed of an asbestos-containing material. The problems with the use of asbestos are well-known. Asbestos fibers and dust are a significant health hazard, and can cause, among others, the conditions known as asbestosis and mesothelioma. Asbestos dust can be generated both during manufacture and during disposal of conventional gassing spiders. Due to worker injury, significant liability for both manufacturers and disposers of asbestos products has become common-place. Accordingly, elimination of asbestos from gassing fuses is desirable.
Thus, what is needed is a spider material which is asbestos free, but which nevertheless functions to quench the arc.